Between pop up thunderstorms, people going on vacation, and
people just not mowing enough, gray leaf spot has exploded on St. Augustine
grass.
Gray leaf spot (Pyricularia grisea) goes with St. Augustine like grits
go with shrimp! Or like chinch bugs go with St. Augustine! To battle gray leaf spot you
are best employing many cultural practices and using limited control products
if necessary.
Gray leaf spot looks like someone burned or dripped acid on
the leaves of the plant. There are
little oblong spots on the leaf.
Eventually, these spots grow together and the leaf blade dies. Whole areas of your grass can disappear at
once when these leaf blades die.
Culturally there are several things to do to minimize your
problem with gray leaf spot. This
disease likes high humidity and excessive nitrogen fertilizer. To help alleviate the high humidity, mow your
grass to a level that seems abnormal to St.
Augustine. Try
to get it down to 2 ½ - 3 inches depending on the variety of St. Augustine grass. Also try to mow every 3 – 5 days with a
bagger. This mowing will help get
sunlight down to the crown of the plant, drying the leaf blades as quickly as
possible.
Mowing is always hard to get someone to modify the frequency
of cuts. “The landscaper only comes every 7 or 14 days”, is what I usually
hear. If you don’t want to cut it yourself in between visits by the landscaper
(saving a trip to the gym and I have been told a cold beer is extra good after
mowing), maybe you could hire a neighborhood kid or your kid to just mow the
grass. No edging, blowing or weeding, just a quick mow.
Another alternative to you mowing is applying a growth
regulator to your turf. These products work great at slowing your turf’s growth
rate. Growth regulators are used extensively on golf courses and athletic
fields. Growth regulators do a great job of managing your turf’s growth rate on
your home lawn as well.
This fungus like most fungi likes hot humid weather. Minimize the amount you water as much as
possible. Wait until your lawn is
getting a blue/green color and your foot prints stay in the lawn after you walk
across it before you water. Unfortunately, you can not control rainfall as
easily. In the Lowcountry, afternoon thunderstorms are a way of life, so keep
the grass mowed as low as you can so it will dry out quickly.
Hold off of the nitrogen fertilizer until you can get this
disease under control. If you need some
color, you could add a product like Possum’s Minors to give you some green
without all the nitrogen. Watch starving your grass because a malnourished yard
is more susceptible to disease. Very low rates of an organic fertilizer or
cotton burr compost might help it recover.
At Possum’s, I know we have had several customers that swear
they control this disease by using our wetting agent with biostimulants, cotton
burr compost, and / or SeaHume along with the above cultural practices.
If you have to resort to a control product, make sure the
product is labeled for Pyricularia grisea.
There are many leaf spot diseases on labels of control products but only
certain ones work on gray leaf spot on St.
Augustine. We had one customer come in that had been
applying a product that controlled Drechslera spp. and Biopolaris spp. leaf
spot; however, the product was not labeled for Pyricularia grisea (watch where
you shop).
Honor Guard and Heritage are systemic products that you
spray. Since this is a leaf spot fungus, the sprays seem to give good coverage
over the leaf blade. If you insist on a
granular product, Prophesy (same active as Honor Guard and Banner), or Disarm are
granular systemic products that are absorbed through the roots and will do a
good job for you.
